Shounen trope of the true villain being the human king. Well this is a shounen so what was I expecting, lul.
Mild rant/brainless mind fodder to get out of my head. Demon Lord shouldn't inherently be evil. It's a title. That's like calling a monarch "Human Lord." Though I do get the natural fear of the unknown, and calling everything "not human" as demonic/evil when they look, well, as such. Would the Demon Lord call the enemy kingdoms King "Human lord?" Would they and their citizens refer to them as such? Demons would have minotaurs, Goblins, and a wide variety of "monster" races. So I suppose the opposite would be the stereotypical Dwarves, Elves, Halflings, half-beasts, etc. Actually, thinking about it even further, the "Demon" lord would be of the demon race. Monster's would fall under their own category of races, just as Humans and Dwarves and so on are separate. So just like how a fantasy king may unite a variety of human-ish races, the demon lord would unite demon-ish races.
Writing can do whatever the author wants though. There is inherent "Connotation" to take into account too. The word "Demon" is a pretty negative connotation or so I would assume. As writers have also done, Demon's and monster's would actually be "demons and monsters." IE, instinctual or big in the killing department. This thought bubble got nowhere.
Well basically. I also kinda wanna see a series where both sides (humans/Demons) have both "good" and "bad" people. Time to write my own Fantasy/Isekai! (which'll be utterly garbage since I'm a total newbie).